1. Field of the Invention
The present invention relates to an electrophotographic photoconductor used in image formation of electrostatic copying processes in, for example, copiers, facsimiles and printers; and to a process cartridge and an image forming apparatus which contain the electrophotographic photoconductor.
2. Description of the Related Art
Inorganic photoconductors made of selenium, zinc oxide, cadmium sulfide, etc., were mainly applied to electrophotographic image forming apparatus such as copiers and laser printers. Organic photoconductors (OPCs), currently, account for nearly 100% of the total production of electrophotographic photoconductors, since they are more advantageous than inorganic photoconductors in reducing production cost, increasing high design flexibility, and giving low environment load. Manufacturers are required to more deeply consider global environmental protection, and to manufacture organic photoconductors (OPCs) as one of mechanical parts instead of a disposable product.
In an attempt to manufacture organic photoconductors (OPCs) having higher durability, there have been taken the following measures: use of a different type of binder resin (see “Hiroyuki Tamura, Saeko Takahashi, Hironobu Morishita, Hideharu Sakamoto, Haruo Shikuma, Japan Hardcopy '97 Fall Meeting, 25-28, 1997”), use of a high-molecular-weight charge transport material (see Japanese Patent Application Laid-Open (JP-A) No. 07-325409), coating of a curable protective layer containing high-hardness fillers (see JP-A No. 2002-258499), formation of a crosslinked resin layer on the surface of a photoconductor (see JP-A No. 2000-66424), and formation of a sol-gel curable film on the surface of a photoconductor (see JP-A No. 2000-171990).
Among the aforementioned measures, formation of a crosslinked resin layer (crosslinked surface layer) can be considered a reasonable solution. This is because the crosslinked layer has a plurality of chemical bonds and, even when stress is applied to cleave some chemical bonds, the layer is not worn in short time.
Organic photoconductors having very high wear resistance must be resistant to scratch formation. This is because, when the surface of such organic photoconductors is scratched, discharge hazard during electrophotographic processes occurs intensively in the scratched portions, resulting in deterioration of these portions. In addition, when the scratched portions (grooves) are embedded with a toner particles-containing developer or paper dust, image failures such as background smear and image blur tend to locally occur. In accordance with improvement in ablation resistance of the photoconductor surface, the formed scratches are difficult to disappear even as time passes (as if they are engraved). Thus, the scratches shorten the service life of the photoconductors.
The recent full-color electrophotographic image forming apparatus generally use polymerization toner, since it provides high-quality images and has high environmental stability. As polymerization toner particles become more spherical, the formed images have an increased sharpness, but toner particles are easier to run through the gap between the photoconductor and the cleaning blade for recovering them. When the cleaning blade is brought into contact with the photoconductor at higher contact pressure, toner particles are prevented from running through the gap therebetween. Meanwhile, the cleaning blade is chipped through acceleration of wear of the contact portion. This results in streaking the photoconductor surface (cleaning failure), and the photoconductor cannot maintain its wear resistance over a long period of time.
The above-described crosslinked surface layer is particularly preferably a cured acrylic film, since it has high wear resistance. However, when used in image forming apparatus using polymerization toner, the cured acrylic film give great damage to the cleaning blade. As a result, the service life of the photoconductor depends on that of the cleaning blade, which is problematic.
JP-A No. 2006-010963 discloses that an electrophotographic photoconductor is provided with a crosslinked surface layer by performing, in combination, thermal curing and photo curing. As described in paragraph [0025] of this literature, use of photopolymerization and thermopolymerization initiators provides a crosslinked surface layer having more uniform cure degree. This crosslinked surface layer does not generate wrinkles and cracks. Also, in this layer, oxidation gas is prevented from adsorbing to unreacted carbon-carbon double bonds.
JP-A No. 2005-077947 discloses that a crosslinked film is formed from a composition containing SUPER BECKAMINE G-821-60 (melamine; product of Dainippon Ink and Chemicals Inc.), Sumidule HT (adduct of HDI isocyanate and trimethylolpropane; product of Sumitomo Bayer Urethane Company Ltd.), and a radical-polymerizable monomer. The crosslinked film exhibits improved resistance to crack formation and exfoliation.
The mechanical strength of the crosslinked surface layer depends on the total chemical bond energy and the crosslink density, and these factors vary with the type of materials used. Thus, demand has arisen for further improvement and development of the crosslinked surface layer.